The present invention relates to a process for the electrochemical graining of aluminum for use in printing plate supports, the process being performed by means of a direct current in an electrolyte containing nitrate ions.
Printing plates (this term referring to offsetprinting plates, within the scope of the present invention) usually comprise a support and at least one radiation-sensitive (photosensitive) reproduction layer arranged thereon, the layer being applied to the support either by the user (in the case of plates which are not precoated) or by the industrial manufacturer (in the case of precoated plates).
As a layer support material, aluminum or alloys thereof have gained general acceptance in the field of printing plates. In principle, it is possible to use these supports without modifying pretreatment, but they are generally modified in or on their surfaces, for example, by a mechanical, chemical and/or electrochemical roughening process (also called graining or etching in the literature), a chemical or electrochemical oxidation process and/or a treatment with hydrophilizing agents. In modern continuously working high-speed equipment employed by the manufacturers of printing plate supports and/or precoated printing plates, a combination of the aforementioned modifying methods is frequently used, particularly a combination of electrochemical graining and anodic oxidation, optionally followed by a hydrophilizing step.
Graining is, for example, carried out in aqueous acids, such as aqueous solutions of HCl or HNO.sub.3 or in aqueous salt solutions, such as aqueous solutions of NaCl or Al(NO.sub.3).sub.3, using an alternating current. The peak-to-valley heights (specified, for example, as mean peak-to-valley heights R.sub.z) of the roughened surface thus obtained are in the range from about 1 to 15 .mu.m, particularly in the range from 2 to 8 .mu.m. The peak-to-valley height is determined according to DIN 4768 (in the October 1970 version). The peak-to-valley height R.sub.z is then the arithmetic mean calculated from the individual peak-to-valley height values of five mutually adjacent individual measurement lengths.
Graining is, inter alia, carried out in order to improve the adhesion of the reproduction layer to the support and to improve the water/ink balance of the printing form which results from the printing plate upon irradiation (exposure) and development. By irradiating and developing (or decoating, in the case of electrophotographically working reproduction layers), the ink-receptive image areas and the water-retaining non-image areas (generally the bared support surface) in the subsequent printing operation, are produced on the printing plate, and thus the actual printing form is obtained. The final topography of the aluminum surface to be grained is influenced by various parameters. By way of example, the following passages from the literature supply information about these parameters:
The paper "The Alternating Current Etching of Aluminum Lithographic Sheet", by A. J. Dowell, published in Transactions of the Institute of Metal Finishing, 1979, Vol. 57, pages 138 to 144, presents basic comments on the roughening of aluminum in aqueous solutions of hydrochloric acid, based on variations of process parameters and an investigation of the corresponding effects. The electrolyte composition is changed during repeated use of the electrolyte, for example, in view of the H.sup.+ (H.sub.3 O.sup.+) ion concentration (measurable by means of the pH) and in view of the Al.sup.3+ ion concentration, with influences on the surface topography being observed. Temperature is varied between 16.degree. C. and 90.degree. C., but does not show an influence causing changes until temperatures are about 50.degree. C. or higher, the influence becoming apparent, for example, as a significant decrease in layer formation on the surface. Graining time is varied between 2 and 25 minutes and leads to an increasing metal dissolution with increasing duration of action. Current density is varied between 2 and 8 A/dm.sup.2 and results in higher roughness values with rising current density. If the acid concentration is in a range from 0.17% to 3.3% of HCl, only negligible changes in pit structure occur between 0.5% and 2% of HCl, whereas below 0.5% of HCl, the surface is only locally attacked, and at the high values, an irregular dissolution of aluminum takes place.
The use of hydrochloric acid or nitric acid as an electrolyte in the graining of aluminum substrates is thus to be considered as being basically known in the art. A uniform graining can be obtained which is appropriate for lithographic plates and is within a useful roughness range. In pure nitric acid electrolytes, adjustment of an even and uniform surface topography is difficult and it is necessary to keep the operating conditions within very close limits.
Other references set forth the influence of the electrolyte composition on the quality of graining. For example, German Offenlegungsschrift No. 22 50 275 (equivalent to British Patent Specification No. 1,400,918) specifies aqueous solutions containing from 1.0% to 1.5% by weight of HNO.sub.3 or from 0.4% to 0.6% by weight of HCl and optionally from 0.4% to 0.6% by weight of H.sub.3 PO.sub.4, for use as electrolytes in the graining of aluminum for printing plate supports, by means of an alternating current. German Offenlegungsschrift No. 28 10 308 (equivalent to U.S. Pat. No. 4,072,589) mentions aqueous solutions containing from 0.2% to 1.0% by weight of HCl and from 0.8% to 6.0% by weight of HNO.sub.3 as electrolytes in the graining of aluminum with an alternating current.
Additives may be used in the HCl electrolyte to prevent an adverse local attack in the form of deep pits. For example, German Offenlegungsschrift No. 28 16 307 (equivalent to U.S. Pat. No. 4,172,772) discloses monocarboxylic acids, such as acetic acid, U.S. Pat. No. 3,963,594 shows gluconic acid, European Patent Application No. 0 036 672 suggests citric acid and malonic acid and U.S. Pat. No. 4,052,275 discloses tartaric acid. All these organic electrolyte components have the disadvantage of being electrochemically unstable and decomposing in the case of a high current load (voltage).
Inhibiting additives, for example, phosphoric acid and chromic acid as described in U.S. Pat. No. 3,887,447 or boric acid as described in German Offenlegungsschrift No. 25 35 142 (equivalent to U.S. Pat. No. 3,980,539) have the disadvantage that there is often a local breakdown of the protective effect and individual, particularly pronounced pits can form in these places.
Japanese Patent Application No. 91 334/78 describes graining by means of an alternating current in a composition comprising hydrochloric acid and an alkalimetal halide to produce a lithographic support material.
German Offenlegungsschrift No. 16 21 115 (equivalent to U.S. Pat. Nos. 3,632,486 and 3,766,043) describes graining by means of a direct current, using dilute hydrofluoric acid, with the web being made the cathode.
German Patent No. 120 061 describes a treatment for generating a hydrophilic layer by the application of electric current, which treatment can also be performed in hydrofluoric acid.
Another known possibility for improving the uniformity of electrochemical graining comprises a modification of the type of electric current employed. For example, an alternating current can be used in which the anodic voltage and the anodic coulombic input are higher than the cathodic voltage and the cathodic coulombic input, the anodic half-cycle period of the alternating current being generally adjusted to be less than the cathodic half-cycle period. This method is referred to in German Offenlegungsschrift No. 26 50 762 (equivalent to U.S. Pat. No. 4,087,341), German Offenlegungsschrift No. 29 12 060 (equivalent to U.S. Pat. No. 4,301,229), German Offenlegungsschrift No. 30 12 135 (equivalent to British Patent Application No. 2,047,274) or German Offenlegungsschrift No. 30 30 815 (equivalent to U.S. Pat. No. 4,272,342).
Another method is to use an alternating current in which the anodic voltage is markedly increased compared with the cathodic voltage, according to German Offenlegungsschrift No. 14 46 026 (equivalent to U.S. Pat. No. 3,193,485).
Another method is to interrupt the current flow for 10 to 120 seconds and re-apply current for 30 to 300 seconds, using an alternating current and, as the electrolyte, an aqueous solution of 0.75 to 2.0 N HCl, with the addition of NaCl or MgCl.sub.2, according to British Patent No. 879,768. A similar process comprising an interruption of current flow in the anodic or cathodic phase is also disclosed in German Offenlegungsschrift No. 30 20 420 (equivalent to U.S. Pat. No. 4,294,672).
The aforementioned methods may lead to relatively uniformly grained aluminum surfaces, but they sometimes require a comparatively great equipment expenditure and, in addition, are applicable only within very closely limited parameters.
Another procedure is a combination of two graining processes. For example, U.S. Pat. No. 3,929,591; British Patent No. 1,582,620; Japanese Patent Application No. 123 204/78; German Offenlegungsschrift No. 30 31 764 (equivalent to British Patent Application No. 2,058,136); German Offenlegungsschrift No. 30 36 174 (equivalent to British Patent Application No. 2,060,923); European Patent Application No. 0,131,926; German Offenlegungsschrift No. 30 12 135 (equivalent to British Patent Application No. 2,047,274) and Japanese Patent Application No. 16 918/82 describe various forms of a combination comprising a pre-texturing treatment which is carried out mechanically in a first step and is optionally followed by chemical cleaning (pickling) and an ensuing electrochemical graining process, in which an optionally modified alternating current is applied in an electrolyte containing hydrochloric acid or nitric acid and which may be followed by an additional cleaning step. All of these references use the advantage of double-graining, which lies, in particular, in a saving of current, but they use an alternating current in the second step.
U.S. Pat. No. 2,344,510 describes the use of graining by means of a direct current in an electrolyte containing hydrochloric acid, which is carried out as a second graining step following a mechanical pre-graining treatment.